Voltage mode, high accuracy battery charger

ABSTRACT

A voltage mode battery charging system is provided that includes a battery current control section, a battery voltage control section, and a power control section. The battery current control section and battery voltage control section each generate signals indicative of the battery charging current and battery voltage, respectively. The power control section generates a signal indicative of the power available from an adapter source. Each of these signals is combined at common node, and if any of these sections exceeds a threshold, battery charging current is decreased.

[0001] The present application claims priority to ProvisionalApplication Serial No. 60/313,260, filed Aug. 17, 2001, and assigned tothe same assignee.

FIELD OF THE INVENTION

[0002] The present invention relates to a battery charger circuit forcharging one or more batteries. In particular, the present inventionrelates to a voltage mode battery charger that uses both current andvoltage control to regulate the charging cycle and to provide accuratecharging and charge termination.

SUMMARY OF THE INVENTION

[0003] In one aspect, the present invention provides a circuit foradjusting the duty cycle of a PWM signal. The circuit includes a batterycurrent control section that generates a current control signalproportional to an amount battery charging current exceeds apredetermined battery charging current threshold. The circuit alsoincludes a battery voltage control section that generates a voltagecontrol signal proportional to an amount a battery voltage exceeds apredetermined battery voltage threshold. A compensation capacitor and acurrent source charging the compensation capacitor are also provided. Acomparator generates a PWM signal based on the amplitude of the voltageon the compensation capacitor. The current source and the currentcontrol signal and voltage control signal are summed together at acommon node, so that the current control signal and/or said voltagecontrol signal reduce the voltage on the compensation capacitor therebyreducing the duty cycle of the PWM signal.

[0004] In another aspect, the present invention provides a batterycharging circuit that includes a current control circuit generating acurrent control signal proportional to the amount battery chargingcurrent exceeds a predetermined battery charging current threshold; avoltage control circuit generating a voltage control signal to theamount battery voltage exceeds a predetermined battery voltagethreshold; a DC/DC converter circuit generating the battery chargingcurrent from a DC source; and a PWM signal generator circuit generatinga PWM signal for controlling the duty cycle of the DC/DC convertercircuit. The PWM circuit comprises a comparator, an oscillator, acompensation capacitor and a current source charging the compensationcapacitor. The comparator generates the PWM signal based on the voltageon the compensation capacitor. The current source and the currentcontrol signal and voltage control signal are summed together at acommon node so that the current control signal and/or the voltagecontrol signal reduce the voltage on the compensation capacitor therebyreducing the duty cycle of the PWM signal and thereby reducing thecurrent delivered by the DC/DC converter circuit.

[0005] It will be appreciated by those skilled in the art that althoughthe following Detailed Description will proceed with reference beingmade to preferred embodiments and methods of use, the present inventionis not intended to be limited to these preferred embodiments and methodsof use. Rather, the present invention is of broad scope and is intendedto be limited as only set forth in the accompanying claims.

[0006] Other features and advantages of the present invention willbecome apparent as the following Detailed Description proceeds, and uponreference to the Drawings, wherein like numerals depict like parts, andwherein:

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a block diagram of an exemplary battery cell chargingsystem according to the present invention;

[0008]FIG. 2 is an exemplary amplifier circuit of the present invention;and

[0009]FIG. 3 is a timing diagram representing an oscillator signal andDC signal to generate a PWM signal of the system of FIG. 1.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0010]FIG. 1 depicts a voltage mode battery charger system 10 accordingto one exemplary embodiment. The system 10 includes a voltage modebattery charger circuit 12 for charging one or more batteries 16 using aDC source 14. The DC source may be an AC/DC adapter or other powersupply. Circuit 12 operates to control the duty cycle of the Buckconverter circuit 18 (comprising an inductor and capacitor, as is wellunderstood in the art) via switches 20, to control the amount ofcharging power delivered to the battery 16. As an overview, circuit 12controls the duty cycle of the Buck converter 18 by monitoring thesource current, the battery charging current (current mode) and thebattery voltage (voltage mode). Battery charging current is sensedacross the sense resistor (or impedance) Rsch. Instead of sensing thecurrent through the inductor (as in conventional current modetopologies), the present invention uses a voltage mode topology bysensing the current across Rsch. In this manner, and by utilizing bothbattery current control and voltage, the present invention achieves moreaccurate charging of the battery towards the end of the charging cycle,and provides more accurate charge termination than can be achieved withconventional current mode charging topologies. The details of the system10 are described below.

[0011] Essentially, the charger circuit 12 operates to control the dutycycle of the buck converter 18 by controlling the power on thecompensation capacitor Ccomp 38. The circuit 12 includes a batterycurrent control section comprised of sense amplifier 26 andtransconductance amplifier 28, a battery voltage control sectioncomprised of summing block 30 and transconductance amplifier 32, and apower control section comprised of sense amplifier 34 andtransconductance amplifier 36. The battery current control section andbattery voltage control section each generate signals indicative of thebattery current and voltage, respectively. The power control sectiongenerates a signal indicative of the power available from the source 14.Each of these sections is combined (at node 60), and if any of thesesections exceeds a threshold, the power delivered to the chargecapacitor decreases, thereby reducing the duty cycle of the Buckconverter. This operation is described in greater detail below.

[0012] The duty cycle of the Buck converter 18 is controlled by thecomparator 40, via switches 20. The input of the comparator 40 is thevoltage on the compensation capacitor (Ccomp) 38 and a sawtooth signalgenerated by the oscillator 44. The output of the comparator 40 is a PWMsignal 68, whose pulse width (duty cycle) is reflected in theintersection of the amplitude of the voltage on Ccomp 38 and thesawtooth signal. In this sense, the duty cycle of the PWM signal thusgenerated is based on the voltage on the compensation capacitor 38 andthe sawtooth signal generated by the oscillator 44. “Based on”, as usedherein, is to be interpreted broadly and generally means “as functionof” or “related to”. The higher the amplitude of the voltage on Ccomp,the greater the duty cycle of the PWM signal 68. In the exemplaryembodiment, the sawtooth signal is a fixed frequency signal, and theduty cycle of the PWM is therefore adjusted by adjusting the amplitudeof the voltage on Ccomp 38. Ccomp 38 is charged by the current source42. When no signal is generated by any of the current control section,the voltage control section or the power control section, the currentsource charges Ccomp to maximum level, and thus the PWM is at maximumduty cycle and the Buck converter is delivering maximum charging currentand voltage to the battery. Any signal generated by the current controlsection, the voltage control section or the power control section actsas a sink to the compensation capacitor 38, thereby reducing the voltageon the compensation capacitor and thereby reducing the duty cycle of thePWM signal. In this manner, charging current is controllably deliveredto the battery 16. The particulars of the Buck converter 18 and switches20 are well understood in this art, and are not important to the presentinvention, and may be generalized as a controllable DC/DC convertercircuit.

[0013] Current Control

[0014] The current control section (circuit) includes a sense amplifier26 and a transconductance amplifier 28. The sense amplifier monitors thebattery charging current across the sense impedance Rsch 24, andgenerates a signal proportional to battery charge current. Thetransconductance amplifier 28 receives the output of the sense amplifier26 and compares that signal with a programmed (desired) battery currentsignal Ich. As a general matter, the inputs of the transconductanceamplifier 28 are voltage signals, and the output is a proportionalcurrent signal. The output of the transconductance amplifier is thecurrent control signal 62, which is proportional to the amount thebattery charging current exceeds the programmed Ich. Ich is zero untilthe battery charging current exceeds the programmed current value Ich.The programmed value Ich is set to according to the particular batterytype and requirements, for example set to charge a conventional LiIonbattery, as is well understood in the art.

[0015] If the battery charging current exceeds the threshold Ich, theamplifier 28 generates a proportional current control signal 62. Sincethe output of the amplifier is coupled to the negative side of thecurrent source 42 (at node 60), any signal generated by the amplifier 28acts to sink current from the source 42. In turn, this operates toreduce the voltage on Ccomp 38, thereby reducing the duty cycle of thePWM signal 68 and reducing the charging current delivered to thebattery. Since the output current control signal 62 is proportional tothe input values, the duty cycle is dynamically adjusted as a functionof battery charging current.

[0016] The current sense amplifier 26 may be a custom or off-the-shelfamplifier, as is readily available in the art. However, as is alsounderstood in the art, amplifier 26 must provide large common modevoltage rejection. Accordingly, and referring now to FIG. 2, anotheraspect of the present invention is an amplifier configuration toalleviate the requirement for large common mode voltage rejection. Thesense amplifier 26 depicted in FIG. 2 includes a switch 48 controlled byan operational amplifier 46, and gain resistors R1 50 and R2 52. Theamplifier 26 of FIG. 2 is not sensitive to common mode voltage. Rather,the switch transfers the floating differential voltage that appearsacross Rsch by referring it to ground and amplifying the voltageaccording to the gain given by R2/R1.

[0017] Voltage Control

[0018] The voltage control section (circuit) includes the summing block30 and a transconductance amplifier 32. In the exemplary embodiment, thesumming block 30 includes three inputs: a high-precision reference ortrim voltage Ref, a voltage set (Vset) and a voltage correction (Vcor)signal. In the exemplary embodiment, the battery 16 is a LiIon battery.LiIon batteries are very sensitive to overvoltage conditions, and indeedbecome hazardous if overcharged. Thus, the reference or trim signal Refis accurate to within the tolerance required by the battery. For LiIon,the tolerance is on the order of +/−0.005 Volts. However, other batterytypes and reference voltage requirements are equally contemplatedherein. Vset represents a voltage setting value, usually supplied by themanufacturer of the battery. Vcor is a correction signal that isproportional to the charging current, and is provided as a compensationsignal for the particulars of the charging apparatus and for parasiticresistance associated with the battery (since battery voltage cannot bemeasured directly, and one must factor in parasitic resistance).Although not shown, Vcor can be obtained by tapping a voltage dividerplaced in parallel with the output of sense amplifier 26. These threesignals are summed in a weighted fashion in summing block 30. Forexample, the output of the summing block 30 can be set to the referencevoltage+(Vset/x)+Vcor/y); where x and y are chosen in accordance withthe desired voltage setting value and correction value, respectively.Vcor and Vset need not be as accurate as the reference voltage, sincetheir contribution is divided diminished by x and y.

[0019] The output weighted voltage signal from the summer block 30 maybe generally deemed as a predetermined battery voltage threshold signal.The transconductance amplifier 32 compares the output of the summerblock to the battery voltage. The output of the amplifier 32 is avoltage control signal 64, which is proportional to the amount thebattery voltage exceeds the threshold established by the summing block.As with the current control section described above, signal 64 isnonzero if the battery voltage exceeds the threshold determined by thesummer block. Since the output of the amplifier 32 is coupled to thenegative side of the current source 42 (at node 60), any signal 64generated by the amplifier 32 acts to sink current from the source. Inturn, this operates to reduce the voltage on Ccomp 38, thereby reducingthe duty cycle of the PWM signal 68 and reducing the charging currentdelivered to the battery. Since the output 64 of the amplifier 32 isproportional to the input values, the duty cycle is dynamically adjustedto achieve a desired battery voltage.

[0020] Power Control

[0021] The power control section (circuit) includes a sense amplifier 34and a transconductance amplifier 36. The power control section isprovided to reduce the duty cycle of the Buck converter, and therebyreduce the charging current delivered to the battery if the DC sourceneeds to deliver more power to an active system 72 (e.g., portableelectronic device) attached to the source. The active system is coupledin parallel to the charging system 10 across the sense resistor Rsac.Since the total amount of power provided by the source 14 is fixed, in awell-designed system the load requirements of the active system andbattery charging circuit are balanced. The power control section ensuresthat the active system always takes priority (in terms of powerrequirements) by reducing the charging current to meet the demands ofthe active system. Accordingly, the power control section generates apower control signal 66 proportional to the amount of power required bythe battery charger and the active system exceeds the threshold Iac_lim.Iac_lim is typically the maximum that can be delivered by the adaptersource 14. For example, the source 14 may be simultaneously supplyingpower to an active system (not shown) and charging current to thebattery. If the portable system requires more power, charging current tothe battery is accordingly reduced to meet the demands of the system.The source 14 is generally defined as a DC power source, as may besupplied from an AC/DC adapter.

[0022] The sense amplifier 34 monitors the total adapter currentdelivered by the source 14 across the sense impedance Rsac 22. The totaladapter (source) current includes the system current (i.e., currentdelivered to a portable system (not shown) connected to the source 14)and the battery charger circuit 12 (which is a measure of the chargingcurrent divided by duty cycle of the Buck converter 18). The signalacross the sense resistor Rsac is a signal proportional to the totaladapter current. The transconductance amplifier 36 receives the outputof the sense amplifier 34 and compares that signal with a powerthreshold signal Iac_lim. Thus, if the signal across the sense resistoris larger than Iac_lim, this indicates that the system is requiring morepower, and accordingly battery charging current is to be reduced. Ofcourse, this limit signal may be fixed, or may be adjusted based on thedynamic power requirements of the system and/or changes in the source.The output of the transconductance amplifier is the power control signal66, which is zero until the power required by the battery charger andthe active system exceeds the threshold value Iac_lim.

[0023] If the power required by the battery charger and the activesystem exceeds the threshold Iac_lim, the amplifier 36 generates aproportional power control signal 66. Since the output of the amplifieris coupled to the negative side of the current source 42 (at node 60),any signal generated by the amplifier 36 acts to sink current from thesource. In turn, this operates to reduce the voltage on Ccomp 38,thereby reducing the duty cycle of the PWM signal 68 and reducing thecharging current delivered to the battery. Since the output 66 of theamplifier 36 is proportional to the input values, the duty cycle isdynamically adjusted as a function of balancing power demands between asystem and the battery, and so as not to exceed a maximum power outputof the DC source 14.

[0024]FIG. 3 depicts a timing diagram 70 representing the PWM signal 68(bottom figure) and the intersection between the voltage on thecompensation capacitor, Vccomp, and the sawtooth signal 44 (top figure).In the present exemplary embodiment, Vccomp is essentially a DC signalwhose amplitude is moved up by the current source 42, and down by eitherthe current control signal 62, the voltage control signal 64 or thepower control signal 66. In other words, the value (amplitude) of Vccompis the sum of signals (42-(62, 64 and/or 66)). By moving the value ofVccomp downward, the duty cycle of PWM signal is decreased.

[0025] Thus, with present invention, the duty cycle of the PWM signalcan be adjusted using a differential the compensation capacitor. In theexemplary embodiments, adjusting the PWM is accomplished dynamically asa function of battery charging current, battery voltage and/or systempower requirements. The topology depicted in FIG. 1 is a voltage modetopology. Voltage mode topology means that the sense resistor Rsch isplaced outside of the Buck converter, and thus the current across thisresistor is a DC value (without ripple). Those skilled in the art willrecognize numerous modifications to the present invention. These and allother modifications as may be apparent to one skilled in the art aredeemed within the spirit and scope of the present invention, only aslimited by the appended claims.

1. A circuit for adjusting the duty cycle of a PWM signal, comprising: abattery current control section generating a current control signalproportional to an amount a battery charging current exceeds apredetermined battery charging current threshold; a battery voltagecontrol section generating a voltage control signal proportional to anamount a battery voltage exceeds a predetermined battery voltagethreshold; a compensation capacitor and a current source charging saidcompensation capacitor; and a comparator generating a PWM signal basedon the amplitude of the voltage on said compensation capacitor; saidcurrent source and said current control signal and voltage controlsignal summed together at a common node, said current control signaland/or said voltage control signal reducing the voltage on saidcompensation capacitor thereby reducing the duty cycle of said PWMsignal.
 2. A circuit as claimed in claim 1, further comprising a powercontrol section generating a power control signal proportional to anamount of total current that is required by an active system and abattery charger; said current source and said current control signal andvoltage control signal and said power control signal summed together atsaid common node, said current control signal and/or said voltagecontrol signal and/or said power control signal reducing the voltage onsaid compensation capacitor thereby reducing the duty cycle of said PWMsignal.
 3. A circuit as claimed in claim 2, wherein said power controlsection comprises a sense amplifier for sensing the total currentgenerated by said source and generating a signal indicative of saidtotal current generated by DC source, and a transconductance amplifiercomparing said signal indicative of said total current generated by saidsource with a predetermined power threshold signal.
 4. A circuit asclaimed in claim 3, wherein said power control signal has a nonzerovalue if said signal indicative of said total current generated by saidsource exceeds said power threshold signal.
 5. A circuit as claimed inclaim 1, wherein said current control section comprises a senseamplifier for sensing charging current supplied to said battery andgenerating a signal indicative of charging current supplied to saidbattery, and a transconductance amplifier comparing said signalindicative of charging current supplied with a predetermined chargingcurrent signal and generating said current control signal.
 6. A circuitas claimed in claim 5, wherein said current control signal has a nonzerovalue if said signal indicative of charging current supplied to saidbattery exceeds said predetermined charging current signal.
 7. A circuitas claimed in claim 1, wherein said voltage control section comprises asumming block generating a predetermined battery voltage signal, and atransconductance amplifier comparing said signal indicative of batteryvoltage with said predetermined signal and generating said voltagecontrol signal.
 8. A circuit as claimed in claim 7, wherein said summingblock having a first input signal comprising a reference voltage signal,said reference signal being selected in accordance with a thresholdvoltage for said battery, a second input signal comprising a batteryvoltage setting signal, said battery voltage setting signal beinggenerated by said battery, and a third input signal comprising a voltagecorrection signal, said voltage correction signal compensating forparasitic capacitance of said battery, wherein said summing blockproviding a weighted sum of said first, second and third input signalsto generate said battery voltage threshold signal.
 9. A circuit asclaimed in claim 1, further comprising an oscillator generating a fixedfrequency sawtooth signal, said comparator comparing said sawtoothsignal and said amplitude of the charge on said charge capacitor andgenerating said PWM signal having a duty cycle adjusted by saidamplitude of the charge on said charge capacitor.
 10. A circuit asclaimed in claim 1, further comprising a Buck DC/DC converter circuitcoupled to a plurality of switches and a DC power source, said PWMsignal controlling the conduction states of said switches to control theduty cycle of said Buck converter to adjust the amount of chargingcurrent delivered to said battery from said DC power source.
 11. Acircuit as claimed in claim 5, wherein said sense amplifier comprisingan operational amplifier coupled in parallel to a sense resistor, saidoperational amplifier sensing the current through a sense resistor, saidcurrent through said sense resistor indicative of said current suppliedto said battery; a switch coupled between one input of said operationalamplifier and ground, the conduction state of said switch beingcontrolled by the output of said operational amplifier; and first andsecond gain resistors placed between said sense resistor and said oneinput of said operational amplifier, and between said switch and areference node, respectively; wherein said signal indicative saidcharging current supplied to said battery being taken from a nodebetween said second resistor and said switch.
 12. A circuit as claimedin claim 2, wherein said active system comprising a portable computer.13. A circuit as claimed in claim 3, wherein said DC source comprises anAC/DC adapter.
 14. A battery charging circuit, comprising: a currentcontrol circuit generating a current control signal proportional to theamount battery charging current exceeds a predetermined battery chargingcurrent threshold; a voltage control circuit generating a voltagecontrol signal to the amount battery voltage exceeds a predeterminedbattery voltage threshold; a DC/DC converter circuit generating saidbattery charging current from a DC source; a PWM signal generatorcircuit generating a PWM signal for controlling the duty cycle of saidDC/DC converter circuit, said PWM circuit comprising a comparator, anoscillator, a compensation capacitor and a current source charging saidcompensation capacitor; said comparator generating said PWM signal basedon the voltage on said compensation capacitor; said current source andsaid current control signal and voltage control signal summed togetherat a common node, said current control signal and/or said voltagecontrol signal reducing the voltage on said compensation capacitorthereby reducing the duty cycle of said PWM signal and thereby reducingthe current delivered by said DC/DC converter circuit.
 15. A circuit asclaimed in claim 14, further comprising a power control sectiongenerating a power control signal proportional to an amount of totalcurrent that is required by an active system and a battery charger; saidcurrent source and said current control signal and voltage controlsignal and said power control signal summed together at said commonnode, said current control signal and/or said voltage control signaland/or said power control signal reducing the voltage on saidcompensation capacitor thereby reducing the duty cycle of said PWMsignal.
 16. A circuit as claimed in claim 15, wherein said power controlsection comprises a sense amplifier for sensing the total currentgenerated by said source and generating a signal indicative of saidtotal current generated by DC source, and a transconductance amplifiercomparing said signal indicative of said total current generated by saidsource with a predetermined power threshold signal.
 17. A circuit asclaimed in claim 16, wherein said power control signal has a nonzerovalue if said signal indicative of said total current generated by saidsource exceeds said power threshold signal.
 18. A circuit as claimed inclaim 14, wherein said current control section comprises a senseamplifier for sensing charging current supplied to said battery andgenerating a signal indicative of charging current supplied to saidbattery, and a transconductance amplifier comparing said signalindicative of charging current supplied with a predetermined chargingcurrent signal and generating said current control signal.
 19. A circuitas claimed in claim 18, wherein said current control signal has anonzero value if said signal indicative of charging current supplied tosaid battery exceeds said predetermined charging current signal.
 20. Acircuit as claimed in claim 14, wherein said voltage control sectioncomprises a summing block generating a predetermined battery voltagesignal, and a transconductance amplifier comparing said signalindicative of battery voltage with said predetermined signal andgenerating said voltage control signal.
 21. A circuit as claimed inclaim 20, wherein said summing block having a first input signalcomprising a reference voltage signal, said reference signal beingselected in accordance with a threshold voltage for said battery, asecond input signal comprising a battery voltage setting signal, saidbattery voltage setting signal being generated by said battery, and athird input signal comprising a voltage correction signal, said voltagecorrection signal compensating for parasitic capacitance of saidbattery, wherein said summing block providing a weighted sum of saidfirst, second and third input signals to generate said battery voltagethreshold signal.
 22. A circuit as claimed in claim 14, furthercomprising an oscillator generating a fixed frequency sawtooth signal,said comparator comparing said sawtooth signal and said amplitude of thecharge on said charge capacitor and generating said PWM signal having aduty cycle adjusted by said amplitude of the charge on said chargecapacitor.
 23. A circuit as claimed in claim 14, wherein said DC/DCconverter circuit comprises a Buck DC/DC converter circuit coupled to aplurality of switches and a DC power source, said PWM signal controllingthe conduction states of said switches to control the duty cycle of saidBuck converter to adjust the amount of charging current delivered tosaid battery from said DC power source.
 24. A circuit as claimed inclaim 18, wherein said sense amplifier comprising an operationalamplifier coupled in parallel to a sense resistor, said operationalamplifier sensing the current through a sense resistor, said currentthrough said sense resistor indicative of said current supplied to saidbattery; a switch coupled between one input of said operationalamplifier and ground, the conduction state of said switch beingcontrolled by the output of said operational amplifier; and first andsecond gain resistors placed between said sense resistor and said oneinput of said operational amplifier, and between said switch and areference node, respectively; wherein said signal indicative saidcharging current supplied to said battery being taken from a nodebetween said second resistor and said switch.
 25. A circuit as claimedin claim 18, wherein said circuit operates in voltage mode by placingsaid sense resistor in parallel with said DC/DC converter for sensingsaid charging current supplied to said battery.
 26. A circuit as claimedin claim 14, wherein said DC/DC converter circuit comprises a Buckconverter comprising an inductor in parallel with a capacitor.
 27. Acircuit as claimed in claim 15, wherein said active system comprising aportable computer.
 28. A circuit as claimed in claim 14, wherein said DCsource comprises an AC/DC adapter.